Improving ergonomics of purging cached resources on clusters, properly adapting command pattern

[ziollek]

Description

VaaS provides a GUI form and API endpoint that enables purging resources from varnish servers.
Each of the above methods allows purging a single resource per call. Obviously, that approach is highly ineffective when it comes to purging massive amounts of resources. Moreover, the API endpoint does not meet REST best practices. Calling complex commands in the context of the cluster is a good example of a command pattern. Best practices on how to implement this pattern can be found here.

Expected behavior:

1. Expose GUI form for batch purging.
2. Expose API endpoint for batch purging accordingly to command pattern